Abstract: Please visit the following link for more details:http://cmb.physics.wisc.edu/journal/index.html
Please feel free to bring your lunch!
If you have questions or comments about this journal club, would like to propose a topic or volunteer to introduce a paper, please email Le Zhang (lzhang263@wisc.edu)

Abstract: Molecular and polymeric organic materials are promising replacements for the inorganic semiconductors in photovoltaic cells due to their large absorption coefficients and easy processing and deposition procedures. Non-traditional nanostructured devices on inexpensive and arbitrary substrates can be fabricated with high throughput using organic materials, leading to vanishingly low module costs. Recent highlights in incorporating organic dyes into photovoltaics devices of varying architectures will be discussed.

Abstract: In this talk, we will discuss several interesting scenarios in which U(1) gauge fields couple to axions through the Stueckelberg mechanism from a string theory perspective. We will first introduce the milli-charged dark matter scenario in which a small electric charge carried by the dark matter particles arise exclusively through kinetic mixing of the photon with some extra massless U(1)'s. We discuss how this apparent contradiction with the "folk theorem" of quantum gravity (that forbids the existence of non-quantized charges) can be resolved when embedded in string theory. Then we will discuss a related scenario - "Stueckelberg Portal" - in which the sizable interactions between Standard Model (SM) and the hidden sectors through heavy Z' bosons arise from a large U(1) mass mixing induced by the Stueckelberg mechanism. Such models present many interesting phenomenological features (e.g. SUSY mediation, hidden valley, no chiral exotics, etc), and are simple and generic in top-down models. We will see how the Stueckelberg Portal can naturally be implemented in string theory. An explicit intersecting D-brane model will be presented. Finally, we will briefly mention some ongoing work on realizing super-Planckian axion decay constants in string theory.

Abstract: For the past sixty years, researchers have studied the electronic structure of donors in silicon using the effective mass approximation, where electronic states are restricted to the vicinity of silicon's six conduction band minima. Despite including central cell corrections and valley-orbit coupling, effective mass theories to date are typically regarded as phenomenological tools, while more computationally-intensive atomistic simulations are more trustworthy.

Here, we present a fully internally consistent effective mass theory that includes
valley-orbit coupling and relies upon only a few approximations. Inspired by recent density functional theory calculations, we include a tetrahedral central cell correction, variationally matching experimentally measured energy levels of phosphorous donors in silicon. When imposing internal consistency, we find both the form of the central cell and the Bloch functions are critically important to obtaining agreement with experiment.

Within this new effective mass framework, we obtain quantitative agreement with the NEMO 3D tight-binding code when calculating the tunnel coupling energy between two phosphorous donors in silicon, a critical quantity for donor-based quantum information processing. We then use our framework, which is several orders of magnitude faster than comparable atomistic simulations, to exhaustively enumerate tunnel coupling over a ~30 nm cube of donor placements, about 1.3 million distinct placements. This high-throughput approach enables the identifications of regions where the tunnel coupling shows little variation among nearby donor positions with high probability, suggesting the feasibility of realistic devices with regular, controllable properties.

This work was supported by the Laboratory Directed Research and Development program at Sandia National Laboratories. Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000.

Abstract:
Supernovae of all types exhibit time-dependent spectropolarimetric signatures produced primarily by electron scattering. These reveal the presence of aspherical phenomena such as complex velocity structures, changing illumination, and asymmetric morphologies within the ejecta or surrounding circumstellar material. The gradual thinning of the ejecta over time also allows us to probe different scattering regions as the supernova evolves. Interpreting the time variations of spectropolarimetric signatures yields unprecedentedly detailed information about supernova explosion mechanisms, the physical processes that shape the density and velocity distributions of the ejecta and circumstellar material, and the properties of the progenitor star.

I will present an overview of supernova spectropolarimetry, highlighting recent observational and computational results. This versatile technique helps us to constrain explosion mechanisms, connect SNe with their massive progenitors (as well as other high-energy transient phenomena such as GRBs), and investigate the process of stellar evolution in other galaxies.